1. Field of the Invention
The present invention relates to a pixel circuit having an electrooptic element controlled in luminance by a signal line in an organic electroluminescence (EL) display device, liquid crystal display (LCD) device, or other active matrix display device and an interconnect structure and arrangement and circuit in an image display device in which these pixel circuits are arranged in a matrix.
2. Description of the Related Art
In an active matrix type display device, use is made of electrooptic elements such as liquid crystal cells and organic EL elements as display elements of pixels. Among them, an organic EL element has the structure of a layer made of an organic material, that is, an organic layer, sandwiched by electrodes. In this organic EL element, by applying a voltage to the element, electrons are injected from a cathode into the organic layer, holes are injected from an anode into the organic layer, electrons and holes are re-coupled as a result of this, and thereby light is emitted. This organic EL element has the following characteristics:
(1) A luminance of several hundreds to several tens of thousands of cd/m2 is obtained by a low voltage drive of 10V or less, so it is possible to lower the power consumption.
(2) Being a self light emitting element, a contrast of the image is high and a response speed is fast, so the viewability is good, making this suitable for display of moving pictures.
(3) Being a completely solid element having a simple structure, it is possible to raise the reliability of the element and reduce the thickness.
An organic EL display device (hereinafter, described as an “organic EL display”) using organic EL elements having these characteristics as the display elements of pixels is considered promising for next generation flat panel displays.
As the methods for driving an organic EL display, there can be mentioned the simple matrix method and active matrix method. Between these methods, the active matrix method has the following characteristics:
(1) The active matrix method is able to hold the light emission of the organic EL element at each pixel over a one frame period, so is suitable for raising the definition and raising the luminance of an organic EL display.
(2) The method enables preparation of peripheral circuits using thin film transistors on a substrate (panel), so enables simplification of the interface with the outside of the panel and raises the functions of the panel.
In an active matrix type organic EL display, generally use is made of polysilicon thin film transistors (TFTs) using polysilicon as the active layer for the transistors used as the active elements. The reason for this is that a polysilicon TFT has a high drivability and enables design of a small pixel size, so is advantageous for raising the definition.
While polysilicon TFTs have the characteristics explained above, it is widely known that they suffer from a large variation of characteristics. Accordingly, when polysilicon TFTs are used, suppression of the variation in characteristics and compensation for the variation of characteristics of TFTs circuit wise are major issues in active matrix type organic EL displays using the polysilicon TFTs. This is due to the following reasons.
Namely, this is because while a liquid crystal display using liquid crystal cells as the display elements of the pixels is configured to control the luminance data of the pixels by voltage values, an organic EL display is configured to control the luminance data of the pixels by current values.
Here, an active matrix type organic EL display will be explained in brief. FIG. 1 is a diagram of a general active matrix type organic EL display, while FIG. 2 is a circuit diagram of an example of the configuration of a pixel circuit of the active matrix type organic EL display (refer to for example U.S. Pat. No. 5,684,365 and Japanese Unexamined Patent Publication (Kokai) No. 8-234683).
In an active matrix type organic EL display 1, m×n pixel circuits 10 are arranged in a matrix, n number of columns' worth of signal lines SGL1 to SGLn driven by a data driver (DRV) 2 are arranged for each pixel column of the matrix array of these pixel circuits 10, and m number of rows' worth of scanning lines SCNL1 to SCNLm driven by a scan driver (SDRV) 3 are arranged for each pixel row.
Further, the pixel circuit 10 has, as shown in FIG. 2, a p-channel TFT 11, a n-channel TFT 12, a capacitor C11, and a light emitting element 13 made of an organic EL element. In the TFT 11 of each pixel circuit 10, a source is connected to a power supply potential line VCCL, and a gate is connected to a drain of the TFT 12. In the organic EL element 13, an anode is connected to the drain of the TFT 11, and a cathode is connected to a reference potential, for example, a ground potential GND. In the TFTs 12 of the pixel circuits 10, sources are connected to signal lines SGL1 to SGLn of corresponding columns, and gates are connected to the scanning lines SCNL1 to SCNLm of corresponding rows. One end of the capacitor C11 is connected to the power supply potential line VCCL, and the other end thereof is connected to the drain of the TFT 12.
Note that the organic EL element has a rectifying property in many cases, so is sometimes called an organic light emitting diode (OLED). Use is made of the symbol of a diode for the light emitting element in FIG. 2 and other figures, but a rectification property is not always required for the organic EL element in the following explanation.
In a pixel circuit 10 having such a configuration, at a pixel for writing luminance data, the pixel row including that pixel is selected by the scan driver 3 via the scanning line SCNL so that the TFTs 12 of the pixels of that row turn ON. At this time, the luminance data is supplied from the data driver 2 via the signal line SGL in the form of voltage and written into the capacitor C11 for holding the data voltage through the TFT 12. The luminance data written in the capacitor C11 is held over a one field period. This held data voltage is applied to the gate of the TFT 11. By this, the TFT 11 drives the organic EL element 13 by the current according to the held data. At this time, gradations of the organic EL element 13 are expressed by modulating the voltage Vdata (<0) between the gate and source of the TFT 11 held by the capacitor C11.
In general, the luminance Loled of an organic EL element is proportional to the current Ioled flowing through the element. Accordingly, the following equation (1) stands between the luminance Loled and the current Ioled of the organic EL element 13:Loled∝Ioled=k(Vdata−Vth)2  (1)
In Equation (1), k=½·μCox·W/L. Here, μ is the mobility of the carriers of the TFT 11, Cox is a gate capacitance per unit area, W is a gate width of the TFT 11, and L is a gate length of the TFT 11. Accordingly, it is seen that variations of the mobility μ and the threshold voltage Vth (<0) of the TFT 11 exert an influence upon the variation of luminance of the organic EL element 13.
In this case, even for example when writing the same potential Vdata to different pixels, since the threshold value Vth of the TFT 11 varies according to the pixel, the current Ioled flowing through the light emitting element (OLED) 13 varies by a large extent and consequently becomes completely off from the desired value, so it is difficult to expect a high image quality of the display.
A large number of pixel circuits have been proposed in order to alleviate this problem. A representative example is shown in FIG. 3. (See for example U.S. Pat. No. 6,229,506 and FIG. 3 of Japanese Unexamined Patent Publication (Kohyo) No. 2002-514320.)
A pixel circuit 20 of FIG. 3 has a p-channel TFT 21, n-channel TFTs 22 to 24, capacitors C21 and C22, and an organic EL element 25 as a light emitting element. Further, in FIG. 3, SGL indicates a signal line, SCNL indicates a scanning line, AZL indicates an auto zero line, and DRL indicates a drive line. An explanation will be given of the operation of this pixel circuit 20 below by referring to the timing charts shown in FIGS. 4A to 4E.
As shown in FIGS. 4A and 4B, the drive line DRL and the auto zero line AZL are made a high level, and the TFT 22 and TFT 23 are made a conductive state. At this time, the TFT 21 is connected to the light emitting element (OLED) 25 in a diode connected state, therefore a current flows in the TFT 21.
Next, as shown in FIG. 4A, the drive line DRL is made a low level, and the TFT 22 is made a nonconductive state. At this time, when the scanning line SCNL is the high level, the TFT 24 is made conductive as shown in FIG. 4C, and a reference potential Vref is given to the signal line SGL as shown in FIG. 4D. The current flowing in the TFT 21 is shut off, therefore, as shown in FIG. 4E, a gate potential Vg of the TFT 21 rises, but the TFT 21 becomes the nonconductive state at the point of time when the potential rises up to VDD−|Vth|, so the potential is stabilized. This operation will be referred to as an “auto zero operation” below.
As shown in FIGS. 4B and 4D, the auto zero line AZL is made the low level, the TFT 23 is made the nonconductive state, and the potential of the signal line SGL is made a voltage lower than Vref by exactly ΔVdata. The change of this signal line potential lowers the gate potential of the TFT 21 by exactly ΔVg via the capacitor C21.
As shown in FIGS. 4A and 4C, when the scanning line SCNL is made the low level and the TFT 24 is made nonconductive, the drive line DRL is made the high level, the TFT 22 is made conductive, current flows in the TFT 21 and the light emitting element (OLED) 25, and the light emitting element 25 starts light emission.
When it is possible to ignore the parasitic capacitance, ΔVg and the gate potential Vg of the TFT 21 become as follows:ΔVg=ΔVdata×C1/(C1+C2)  (2)Vg=Vcc−|Vth|−ΔVdata×C1/(c1+C2)  (3)
Here, C1 indicates the capacitance value of the capacitor C21, and C2 indicates the capacitance value of the capacitor C22.
On the other hand, when the current flowing in the light emitting element (OLED) 25 at the time of the light emission is Ioled, the current value of this is controlled by the TFT 21 connected in series to the light emitting element 25. Assuming that the TFT 21 is operating in a saturated region, the following relationship is obtained by a well known equation of a MOS transistor and the above Equation (3):Ioled=μCoxW/L/2(Vcc−Vg−|Vth|)2=μCoxW/L/2(ΔVdata×C1/(C1+C2))2  (4)
Here, μ indicates the mobility of the carriers, Cox indicates the gate capacitance per unit area, W indicates the gate width, and L indicates the gate length.
According to Equation (4), Ioled is not controlled according to the threshold value Vth of the TFT 21, but by ΔVdata given from the outside. In other words, when the pixel circuit 20 of FIG. 3 is used, it is possible to realize a display device having a relatively high uniformity of the current and consequently uniformity of the luminance without influence of the threshold value Vth which varies for each pixel.